Shoot the slug gun a little. The reason I say this is because I do the same but for a different reason. I travel about 250 miles from home to deer hunt. I intend to hunt with my muzzleloader but always take one of my slug guns for a extra firearm incase I have problems with my muzzleloader or it's raining. But to answer your question I don't think you would benefit much shooting a slug gun to "practice up" for shooting your muzzleloader. They are two differnet beasts. You would be better off shooting a .22 rifle and then go to your muzzleloader. Just my opinion.